1. Field of the Invention
The present invention relates to systems and methods for post-processing decompressed images in order to minimize perceptual artifacts due to prior image compression, and in particular to such prior image compression methods that process images as independent blocks of pixels.
2. Description of the Related Art
Many important image compression methods process images as independent blocks of pixels. For example, such families of compression standards as JPEG, MPEG, H.320, and so forth, specify a step involving discrete cosine transformation ("DCT") of independent, non-overlapping 8.times.8 blocks of pixels in the source image followed by quantization of the resulting transform coefficients. See, e.g., Jack, 1996, Video Demystified, HighText Interactive Inc., San Diego, Calif. The quantized transform coefficients are transmitted from a transmitter-encoder to a receiver-decoder. Such transformation and quantization together achieve compression by exploiting the significant correlations that typically occur between the values of pixels in 8.times.8 blocks, but result in loss of image information ("lossy" compression), the coarser the quantization the greater the loss.
Decompressing images so compressed, which necessarily involves steps of dequantization and inverse DCT of the received quantized coefficients in order to derive a received decompressed image, can lead to what are called herein "blocking artifacts" in the following manner. In certain areas of a received image, the quantization errors introduced can become especially apparent and even objectionable. Especially, in regions where the image is fairly smooth, with little high spatial frequencies components, errors in the low spatial frequency components can make the individual, independent 8.times.8 blocks perceptually apparent. This is especially so if low frequency components, which smoothed the block-to-block boundaries in the source image, are set to zero.
Several methods of reducing such blocking artifacts are available in the current state of the art. Simple lowpass filtering applied to the decompressed image can blur blocking artifacts and reduce their prominence to some extent, but it necessarily leads also to an overall degraded sharpness in the image. Blocks can be overlapped in the source image in order to redundantly encode block-to-block boundaries, but at the cost of decreased compression and increased required communication bandwidth.
Further, Pennebaker et al., 1993, JPEG Still Image Compression, Van Nostrand Reinhard, chap. 16, discloses JPEG block smoothing by fitting quadratic surfaces to the average values of pixels (equivalent to the "DC", or lowest order, transform coefficient) in adjacent blocks, a computationally complex process. Lakhani, 1996, "Improved Image Reproduction from DC Components", Opt. Eng. 35:3449-2452, discloses equations for predicting low frequency transform coefficients from DC coefficients that are improved from those in the JPEG standard. Finally, Jeon et al., 1995, Blocking Artifacts Reduction in Image Coding Based on Minimum Block Boundary Discontinuity, Proc SPIE 2501:189-209, discloses a complex and computationally expensive iterative method for zeroing block boundary discontinuities.
Importantly, all current art methods appear to achieve blocking artifact reduction by in one fashion or another performing versions of spatial low-pass filtering. These current art methods also all suffer from one or more additional problems, such as producing overall image degradation, limiting image compression, failing to explicitly address the perceptual aspects of blocking artifacts, requiring excessive computational resources, and so forth.
What is needed, therefore, is a method and system for post-processing decompressed images which is computationally efficient, avoids spatial low-pass filtering, does not produce image degradation, has no effect on compression, and, most importantly, minimizes the perceptual aspects of blocking artifacts.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.